I have Arma2 Free and Operation Arrowhead via Steam, so these instructions would be particular to the steam version I suppose.

1    Download and run/install Operation Arrowhead.

2    Download and run/install Arma 2 Free

3    Download the mod files and extract them to a common location

4    Go to your steam folder/steamapps/common/arma 2 operation arrowhead folder. Inside this folder, create a new folder called : "@DayZ" without the quotes

5    Inside the @DayZ folder, create a new folder called: "Addons" again without the quotes

6    Inside this Addons folder, paste the extracted mod files.

7    Go back to the Arma 2 OA folder. Create another new folder called: "Arma2"

8    Go to your steam arma 2 free folder (same general location as OA folder). Copy all of the contents inside, go back to the Arma 2 OA folder, and paste all those things into the Arma2 folder you just created.

9    Create a desktop shortcut of the file ArmA2OA.exe

 10   Right click this shortcut, click properties, and in the target box after the quotations paste: -mod=Arma2;EXPANSION;CA -beta=Expansion\beta;Expansion\beta\Expansion -mod=@DayZ -nosplash

For example, mine looks like this: "C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead\ArmA2OA.exe" -mod=Arma2;EXPANSION;CA -beta=Expansion\beta;Expansion\beta\Expansion -mod=@DayZ -nosplash

Game should run then. Only run it from the desktop shortcut